The present invention relates to a method for controlling the combustion of fuel in a recovery boiler, and more particularly, to a method for maximizing the reduction efficiency of said combustion.
A recovery boiler is a furnace wherein a waste fuel and air are combusted and chemicals from the waste fuel is recovered. In the pulp and paper industry one such waste fuel is called black liquor, which comprises in part water and sodium sulfate (Na.sub.2 SO.sub.4). The combustion of black liquor in a recovery boiler results among others in a chemical process in which sodium sulfide (Na.sub.2 S) is recovered through the chemical reaction of the combustion process. In the pulp and paper industry, the recovery of sodium sulfide is essential to the paper manufacturer, inasmuch as the chemical is used in the chemical reaction to break the lignin of the fibers to produce pulp. For the pulp and paper industry then, the recovery boiler serves two functions, viz. an essential chemical of the paper producing process is produced from the recovery boiler, and a certain amount of energy is liberated for use to generate steam and/or electricity for use at the mill.
A recovery boiler comprises a fuel input, and a plurality of air inputs, a smelt output and an exhaust output. The air input, which is closest to the bed of the boiler in which air enters into the recovery boiler, is termed the primary air input. In the sequence of order of location of other air inputs into the boiler, the other air inputs into the boiler which are in successive further distance away from the bed are termed secondary air inputs and tertiary air inputs respectively. Fuel and air are primarily combusted in a zone which is located near the level of the secondary air input, and referred to as the oxidation zone. It has long been recognized that the primary air input is responsible for controlling the amount of air entering into the area just above the bed of the boiler, hence for creating either a reducing atmosphere or an oxidizing atmosphere in the area just above the bed of the boiler (a reducing atmosphere being defined as an oxygen starved atmosphere, whereas an oxidizing atmosphere is defined as an oxygen enriched atmosphere). The combustion of black liquor in a recovery boiler in a reducing atmosphere results in the following main chemical reaction: EQU 2C+Na.sub.2 SO.sub.4 .fwdarw.2CO.sub.2 +Na.sub.2 S
The molten state of sodium sulfide (Na.sub.2 S) which is recovered from the bed of the boiler is termed smelt. It has been recognized that for this chemical reaction to take place, a reducing atmosphere should be maintained in the area just above the bed, hereafter referred to as the reduction zone. If there is too much primary air above the bed, then the reduction efficiency is decreased since an oxidation reaction instead of a reduction reaction will take place. Moreover, the heat released by the oxidation (combustion process) will primarily be used to raise the temperature of the excess amount of primary air. The raising of the temperature of the excess amount of primary air will cause a large upward draft of air. The upward draft will cause the liquor droplets to be retained longer before hitting the bed. The longer the liquor droplets remain in its flight, the more water in the liquor will evaporate and the combustion process will have to proceed further prior to the liquor droplets hitting the smelt bed. These effects will result in a gradually cooling surface temperature of the bed leading toward an eventual extinction of the fire. However, on the other hand, if too little primary air is supplied, the combustion process will not proceed causing the temperature in the smelt to decrease making it difficult to drain. The bed will then start building up, increasing the rate of cooling and rapidly extinguishing the fire. Hence, a very critical measure of the performance of this zone is the temperature above the bed.
Heretofore, one method of measuring the temperature above the bed is to take a direct measurement of the temperature of the bed through an optical pyrometer. While this direct approach is in theory the best, practical implementation of this approach has led to many difficulties due in part to (1) the temperature of the bed which is at an extremely high temperature, typically on the order of one thousand degrees centigrade (1000.degree. C.) necessitating cooling means for the pyrometer; and (2) the dirty environment in which the pyrometer must operate and thus, it is subject to reliability problems.